Elasticsearch uses a custom discovery implementation called "Zen Discovery" for
node-to-node clustering and master election. There are two important discovery
settings that should be configured before going to production.

Out of the box, without any network configuration, Elasticsearch will bind to
the available loopback addresses and will scan ports 9300 to 9305 to try to
connect to other nodes running on the same server. This provides an auto-
clustering experience without having to do any configuration.

When the moment comes to form a cluster with nodes on other servers, you have to
provide a seed list of other nodes in the cluster that are likely to be live and
contactable. This can be specified as follows:

To prevent data loss, it is vital to configure the
discovery.zen.minimum_master_nodes setting so that each master-eligible node
knows the minimum number of master-eligible nodes that must be visible in
order to form a cluster.

Without this setting, a cluster that suffers a network failure is at risk of
having the cluster split into two independent clusters — a split brain — which
will lead to data loss. A more detailed explanation is provided in
Avoiding split brain with minimum_master_nodesedit.

To avoid a split brain, this setting should be set to a quorum of
master-eligible nodes:

(master_eligible_nodes / 2) + 1

In other words, if there are three master-eligible nodes, then minimum master
nodes should be set to (3 / 2) + 1 or 2: